Independence Day Resurgence

We finally got around to watching Independence Day Resurgence... there was a lot of lazer beams, explosions, speeches and explosions.

But we did find out the reason why the aliens attack other planets. They want to harvest the molten core. In the 2nd film they send a 3000 mile ship to do just that. And nearly succeed. I get it.

OK, so why didn't they just do that in the first movie?

I mean, they blew up all the major planetary monuments, killed loads of people and created some interesting special effects. But why? If they're only after the core and we can't even penetrate their shields, why bother messing around? Why not just go right for the core?

Now, I get it. When they made the 2nd movie someone forgot to watch the original. To ensure continuity and all that...